More than thirty years ago, horizontal roofs were common in commercial buildings having a raised peripheral ridge, but being slightly sloped toward a drainpipe projecting transversely from the roof. This drain was designed to collect rainwater or melting snow which would build up on the roof. However, for a variety of reasons, this roof was not totally satisfactory. Henceforth, the considerable use of concrete and metallic structures for buildings, in Canada and the United States, has enabled use of completely flat roofs, i.e. having a zero slope i.e. completely horizontal.
In these reinforced structures, the building upper frame often consists of a steel planking which is not well suited to the installation of a uniformly fluid-tight membrane. In view of resolving this problem, experts have devised the addition of a layer of gypsum, mechanically secured to the steel planking so as to thus offer a uniformly uninterrupted surface to support a lower fluid-tight membrane which receives a warm asphalt layer so as to thus become a fluid-tight membrane, preventing any water or water vapour seeping from the interior to the exterior and vice-versa. On this lower membrane, insulating panels are installed when the asphalt is still warm. A wood fibre support plank is then applied against the insulating panels, so as to provide a uniform, flat support for the upper watertight membrane. A surface drainpipe permits the discharge of surface water from the roofing.
With time and wear, it will be understood that the upper membrane will eventually rupture itself much sooner than the lower membrane. Water will seep in between the two membranes. Since the lower membrane remains watertight, water will accumulate between the two membranes soaking the insulating panels and decreasing their heat-insulating efficiency.
Water build-up between the two membranes is concealed from view. Therefore, it is not possible to verify de visu if there is good roof draining. The building structure is then susceptible to experience unexpected loads exceeding tolerance levels.
Then, one should not exclude the possibility of roof collapse, with all the material and human risks involved. This is the more so when considering computations made by the two present joint-inventors, which have revealed that such overload values will exceed the safety margins generally allowed for the construction of flat roof buildings. PG,5